The discrepancy between the observed particle's mean free path against scattering &lgr;fit as derived from applying time-dependent spatial diffusion models to solar particle events and the theoretical value &lgr;t from quasi-linear theory can be resolved. A more complete quasi-linear result for the pitch-angle diffusion coefficient of particles in an uniform magnetic field with superposed parallel and antiparallel propagating transverse Alfv¿n waves yields finite and well-behaved expressions for the mean free path &lgr;t even for rather large values of the spectral index q of the assumed power law spectrum of the power spectrum of magnetic irregularities. It is shown that values of q between 2.1 And 2.2 lead to remarkable good agreement of &lgr;t with the observed variation of &lgr;fit from solar cosmic ray events. ¿ American Geophysical Union 1988